1. Field of the Invention
The present invention relates to devices for filtering electrical stimultion interference from physiologic monitoring devices.
2. Prior Art
Electrical stimulation is well accepted as a means of pain control. Stimulation is provided either externally through transcutaneous electrical nerve stimulation (TENS) or internally through implanted devices such as spinal cord stimulators.
There are many situations in which this method of pain relief is not available because it would interfere with electrical signals being sensed on the body. In these situations, the patient must resort to less desirable pain control means, such as drug therapy.
For example, one situation in which TENS is used to control pain is during post-operative recovery. In this situation, there is often constant EKG monitoring of the patient. TENS cannot be used while EKG is being monitored, since the TENS interference will distort the EKG signal.
EKG devices reject noise which is common to both leads. In TENS, however, the TENS stimulation signal will appear at different levels on the two leads through which the EKG is being sensed. Therefore, a distorted signal will be displayed on the EKG monitor.
There have been various attempts to use TENS while monitoring patients. In U.S. Pat. No. 4,161,945 to Grossman, a selective interference filter is employed. This involves band rejection or a notch filter to filter the bandwidth in which TENS stimulation would normally occur. This necessarily distorts the signal in that the bandwidth which is filtered must be altered in the EKG signal.
U.S. Pat. No. 4,331,157 to Keller, Jr. et al, discloses apparatus for temporarily holding EKG signals at such time as TENS stimulation is occurring. Therefore, the two can coexist, although not stimultaneously.
These prior art attempts fail to allow simultaneous stimulation and monitoring, without degrading the monitored signal.